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Unformatted text preview: ‘ Name a j CHE/ME 109 Exam II Section II ' Please CIRCLE your answers. Good luck! . _ . ' I V ' ‘ IV I » I I ProblemI '   ’_ 1 _ is  l' I" _ > 5 pts. (a) A hot surface is cooling by natural convection. The surface is 77°C and the air I l of heat transfer? Be sure to igclude the proper units. p 1 g
‘ . L ' ﬂ 1  ’ I 571%, w/S ‘ v W. iii... 32 18K k= O’Z:+S L’s/th _ _ V 10 pts. (b) If the surface is vertical and has dimensions of 0.5 m x 0.5 m, determine '
whether the air ﬂow is laminar or turbulent. To receive credit, you need to calculate the appropriate dimensionless number for this problem to justify your answer. * I Chanel—en‘ylx'g 93nd = :5M 3 I l
l I a 4— l.C527</0'5 1441/5)?— __.—————‘——— temperature is 27°C. What are the properties of the air for the determination of the rate ‘ 9 10 pts. (c) If a fan were placed at the base of the surface so that the ﬂow would be up
along the surface vertically, what would be the minimum velocity of the fan at which you
could neglect the natural convection heattransfer? i VC‘L Ool 'l’O “4+1de ConVeﬁﬁoy‘ Qa ,_.__/<o. . ‘ ~, 5
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Re: v EL/____S_'__,————:V _ J. a MW j Cenozch‘qn _ s. Problem II. Short answer. ' 5 pts. 1. When is heat transfer through aﬂuid conduction and when is it convection? 14‘ m “wick is moving 1+ wlu Imme , COHVCC‘hbn Wi‘ MIAS‘CCJ: H can ' ' I move, \0 Moﬁ‘hn of by > bwcjﬂrkc; curve/n+5. 5 pts. 2. What is the signiﬁcance of the critical Reynolds Number? I, v m crat‘ml ,Ranchij number V , [(Acitgccnag' 4/141 Mng'Hﬁop a4“ ‘Giow ‘r‘ij'l/VW»  Lou/\Q'V‘CW‘ {*0
hILuU/k't, MA MLCU/ +urloulem+ .v.._.._._~__—...__.——v . . v....‘.r._.. . _ ~‘1J .024 m ' ' Problem In " ' Tu % 27°C Transistor 20 pts. Illa. A commercially available heat sink is used to cool a transistor. If the
surface of the heat sink is the same temperature as the transistor(i.e. the ﬁns are 100%
efficient), determine the maximum power of the transistor if the surface temperature is
not to exceed 77°C. For simplicity, ignore the surface area of the transistor to
calculate the area for heat transfer of the base plate. Calculate the amount of
heat transferred by natural convection. All of the surfaces are vertical hot plates. m a >96¢9K 124.222: >‘> taunt an,
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10 pts. M b. Calculate the amount of heat transferred from the heat sink by radiation.
Consider only the outerfacing sides of the two ﬁns (i.e. use the area for radiation heat a. _ aX Dowager
QﬂA:€UA(E my) {smegma _ . ﬂ 3004) 24,11 \N “Mm ._.—.i . ..__< .u.«w.£.aw.ﬁ_p “_~ ._._._——__._. ._ .._.__.._.—...' _,__.‘ ' '1'! Problem I V. I 15 pts. A SUNNY computer has 1 PCB which generates 80 of power. The air
duct has a cross sectional area of 6.3 x 105 m2 . The maximum temperature of the room air is 35°C and the maximum allowable exit temperature of the air is 38°C. Assume the
air is at 1 atm of pressure. Determine the appropriate volumetric ﬂow rate of the fan to
3 a? + ?5 cool the computer.
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_.._.._.___.._———.—.._..u—"_. . . ... ..._...._.—__...._ .._. lVb. 15 pts. There is one component in the computer that generates 5 W of power. The
component is placed close to the exit of the computer and a heat sink is attached. Ifthe
surface of the component is not to exceed 75°C, calculate the minimum surface area of
the heat sink. Assume the heat sink is 100% efﬁcient. Use 0.003 m2 as the cross
sectional area of the air channel. Ifyou were not able to calculate the volumetric ﬂow rateinparta, use V=2x10~2m3/s, TM cal/mac! . . _
5w abateTwo T5162 newt R>7rg+3g= 330K
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 Spring '08
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 Heat Transfer, pts, heat sink

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